Conventional wideband code division multiple access (WCDMA) low noise amplifier (LNA) and mixer architectures have an external surface acoustical wave (SAW) filter between the LNA and mixer. The purpose of the SAW-filter is to attenuate the transmit (TX)-signal that leaks into the LNA through the finite isolation of the duplexer.
FIG. 1 shows typical architecture included in a mobile station, such as a WCDMA handset. The architecture includes a transmitter 4 (TX) having an output connected to a power amplifier (PA) 5, which amplifies the transmit signal output. The output of the PA 5 is connected to the duplexer 6. Also connected to the duplexer 6 is a receiver 7 (RX) and antenna 8. The receiver 7 includes on-chip components 9 as well as off-chip components. While direct conversion architecture provides a way to integrate many components of the receiver 7, such as the low noise amplifier 12, in-phase (I) mixer 14, quadrature (Q) mixer 16, local oscillator (LO) 18, phase shifter 19, baseband channel filters including a low pass filter 20 for the I signal and a low pass filter 22 for the Q signal, and variable gain amplifiers (VGAs), the SAW-filter is an off-chip component. In FIG. 1, the SAW-filter is depicted as band pass filter (BPF) 10.
The duplexer 6 typically provides some 50-55 dB isolation from TX to (receive) RX. However, additional isolation is normally needed to prevent the TX-leakage from deteriorating the receiver's performance. The receiver is degraded through intermodulation generated by second and third order distortion. For a circuit supporting multiband WCDMA, the solution with the SAW-filter 10 between the LNA 12 and mixers 14 and 16 becomes troublesome because each supported band would need its own SAW-filter. However, such a solution would be unattractive because it would add cost and printed circuit board (PCB) area.
There are several possible combinations of single ended/differential LNA and mixers that could be used in a SAW-less architecture:
Differential LNA and differential mixer: The drawback is the additional package pin for the LNA. A multiband circuit will need a larger package.
Single ended LNA and differential mixer. The drawback is the large on-chip balun needed between the LNA and mixer. If several baluns are needed in a multiband solution, the area penalty is increased.
Single ended LNA and single ended mixer. In an all single ended solution, there is no need for an on-chip balun.
In conventional architectures with a SAW filter present between the LNA and mixer, the SAW filter can have a single ended input and a differential output. The LNA can then be designed with a single ended input and a single ended output connected to the SAW filter. If the SAW filter output is differential, the mixer can be designed as double balanced, i.e., with differential RF input and differential LO input. An alternative architecture may include a SAW filter with both single ended input and output in combination with an on-chip balun to create the differential RF signal.
A WCDMA LNA and mixer are prone to second order intermodulation distortion (IM2) and third order intermodulation distortion (IM3). The IM3 product that the interstage SAW filter helps against is the one that is generated in the mixer transconductance stage and in the switching mixer core. For WCDMA, the worst intermodulation case is when an interferer is present at half the duplex distance between the RX and TX frequency. The third order nonlinearity of the LNA and mixer will create a spurious signal at the RX frequency originating from the TX-leakage into the LNA and the interferer at half the duplex distance, which is illustrated as path 30 in FIG. 1. The SAW filter attenuates the TX-signal and the out of band blocking signals and therefore also the intermodulation product that is generated from these signals.
Example: WCDMA band I with Duplex Distance 190 MHz
    fRX=2140 MHz    fTX=2140 MHz−190 MHz=1950 MHz    f1/2-duplex-interferer=2045 MHz    fIM3=2·2045 MHz−1950 MHz=2140 MHz=fRX 
The LNA and mixer IM2 arises when two interferers at f1 and f2 imputted into a mixer with a second order nonlinearity generate an intermodulation product at their difference frequency f1−f2. This intermodulation product will fall directly into the wanted downconverted baseband frequency band if the interferers are close to each other. In a WCDMA receiver, the worst-case interferer for the mixer second order nonlinearity is the TX-signal that leaks into the receive path through the finite TX-RX isolation of the duplexer. The TX-signal is a WCDMA digitally modulated interferer with AM- and FM modulation. The AM-modulation can be represented by a two-tone interferer with two close frequencies at f1 and f2. The second order nonlinearity of the mixer will translate a squared version of the envelope of the TX-signal to the receiver mixer output. This is the scenario that defines the second order intercept point (IP2) requirement for the LNA and mixer. The receiver IM2 level due to TX-leakage is tested in a 3GPP standard test case that specifies the minimum required sensitivity for bit error rate (BER) <10−3 while the transmitted signal is at maximum power level (+24 dBm) at the antenna.
There are three mechanisms that generate second order distortion in a zero-IF receiver: RF self-mixing, second order nonlinearity in the mixer transconductance stage, and cross modulation of the LO-leakage.
RF self mixing occurs when the RF signal leaks to the LO-signal in the mixer through parasitic coupling in the mixer core switching devices. This leakage is illustrated as path 32 in FIG. 1. If the LO-amplitude is not high enough the mixer behaves more like a linear multiplier and consequently the mixer output will contain a signal that is proportional to the square of the input signal i.e. an IM2 product. If the RF signal is the TX-leakage with AM-modulation, a low baseband frequency IM2 product will be generated through self-mixing in the switching mixer core transistors. However, if the LO-amplitude is high enough this effect is significantly reduced.
A second order nonlinearity also exists in the transconductance transistors that generate the RF-current supplied to the mixer core switching transistors. An AM-modulated interferer represented by two frequencies, f1 and f2, will generate a low frequency second order intermodulation product at f1−f2 that is added to the wanted output current from the transconductance transistor. If the mixer is perfectly balanced, i.e. there is no mismatch in the switching core transistors, the mixer load resistor, or the LO driver block, the low frequency intermodulation product at f1−f2 will not reach the mixer output. In reality, a mismatch inevitably exists in these components. As a result, this intermodulation product leaks to the mixer output.
In a double balanced mixer, the mixer transconductor is a differential stage with built-in suppression of the even order IM2 products. The amount of suppression is dependent on the matching of the devices in both the transconductance stage and the mixer switching core. A solution with a single ended mixer relies on the matching in the mixer switching core only for suppressing IM2 originating from the transconductance stage.
Cross modulation of the LO-leakage is a mechanism through which the AM-modulation of the TX-leakage interferer at the mixer core RF input transfers to the LO-leakage at the mixer RF input. Downconversion of this AM-modulated LO-leakage with the LO-signal itself will generate a mixer output signal at the IM2-frequency. Compared with the IM2 products generated by self-mixing and second order nonlinearity in the mixer transconductance stage, the cross modulation product is a differential signal, i.e., the phase of the IM2 product at the two mixer outputs differ by 180 degrees.